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IP Phone CoS - Urgent :(

Hello, I just need to confirm a couple of C0S points.

I have an IP Phone into a c3550 with a PC off the ip phone

This example shows how to configure the default port CoS to 4:

Switch(config)# interface gigabitethernet0/1

Switch(config-if)# mls qos trust cos

Switch(config-if)# mls qos cos 4

This example shows how to assign all the packets entering a port to the

default port CoS value of 4:

Switch(config)# interface gigabitethernet0/1

Switch(config-if)# mls qos cos 4

Switch(config-if)# mls qos cos override

Can I just confirm that example one, leaves the trusted CoS values (say from an IP phone) in tact, and any untagged (ie DATA) frames are set to a C0S of 4.

In the second, all packets regales if trusted are overwritten with the default cos vlaue.


If the first example is correct, and all the voice CoS are kept in tact, if i want to change the CoS of data frames, which is the best way to do this :-

Can I do this two ways.

int fa 0/2

switchport priority extend cos <value>

int fa 0/2

mls qos cos <value>

I undertsand the first to have the IP phone set the CoS and the second, the switchport sets the CoS value for all data "UNTAGGED" frames.


Re: IP Phone CoS - Urgent :(

IP phones classify their voice and control traffic at Layer2 (802.1Q/p) and at Layer3(DSCP).

The recommended configuration is to trust the L2 (CoS) classification applied by the phone on the access port servicing the phone.

This is accomplished through the application of the trust cos port configuration option. Although the IP phone will classify traffic from the PC port with a cos 0, setting at the access-port using switchport priority extend cos 0 has the same desired affect.

The following is an example of a port configuration, which will trust both RTP and Control traffic from the IP phone.

int fa 0/1

description DOT1Q port to IP Phone

switchport trunk encapsulation dot1q

switchport trunk allowed vlan ,

switchport mode trunk

switchport voice vlan

mls qos trust cos

no snmp trap link-status

spanning-tree portfast

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